This invention relates in general to a magnetic disc unit system and more particularly to a magnetic disc unit suitable for changing a cylinder address designated by a host unit into a different cylinder address by maintaining correspondence with the physical (cylinder) address associated with the magnetic disc.
Due to the development of information processing systems, widespread use has been made of the magnetic disc, particularly, for storing therein an operating system program. The magnetic disc unit has a plurality of such magnetic discs which are stacked vertically, for example. Each magnetic disc 41 has a magnetic disc surface as shown in FIG. 1. More particularly, the magnetic disc surface is formed, at its innermost and outermost circumferences, with guard portions respectively termed an inner guard band (IGB) and an outer guard band (OGB) and with an intermediate portion between the inner and outer bands which is allotted with cylinder addresses 0, 1, 2, . . . , N-3, N-2 and N-1 (N is an integer) sequentially numbered from the outer to the inner circumferences. The magnetic disc unit includes an access mechanism which is responsive to a cylinder address instructed by the host unit to move the magnetic head to a position corresponding to the instructed cylinder address, for implementation of a read/write processing of data.
The process for movement of the magnetic head, i.e., seek operation will be explained with reference to a flow chart shown in FIG. 2. In a step 101, the magnetic disc unit is powered-up (power on) and first initialized. In a step 102, a rezero operation is implemented to return the magnetic head to a position corresponding to the cylinder address "0". Subsequently, in a step 103, "000" representing the cylinder address for the present position is set in an internal register A.sub.Reg. The rezero operation is then carried out. In the rezero operation, the present head position is detected by moving the head to the outer area of the cylinder address area and reading out of the guard band information recorded on the inner guard band IGB and the outer guard band OGB so that the magnetic head is positioned on the boundary position between the OGB and the cylinder address area, that is cylinder address "0". Once the rezero operation is completed, a seek operation can be implemented such that the number of cylinders through which the magnetic head passes reflects an ultimate cylinder address for the head.
Responsive to a seek instruction from the host unit, an internal register B.sub.Reg is set with a cylinder address representative of a destination, and the difference in the addresses is calculated from the value or contents of register B.sub.Reg and the value of register A.sub.Reg to determine a moving direction and an amount of movement of the magnetic head. The results are set in an internal register C.sub.Reg, and the magnetic head is moved by the access mechanism responsive to the contents of the register C.sub.Reg to reach the destined cylinder position (steps 104 to 107).
As described above, conventionally, the seek operation is controlled using sequential cylinder addresses which are allotted to the magnetic disc 41. Accordingly, if a change in the correspondence of a cylinder address with the physical position of the magnetic head corresponding thereto is needed because of, for example, an occurrence of a defective cylinder address, the host unit is then required to change its program for the seek instruction. Further, since in conventional magnetic disc units the first cylinder position which the magnetic head initially encounters as it moves from the OGB to the data area in the intermediate portion is defined as cylinder address "0" for seek operation control, it has been impossible to produce a magnetic disc unit capable for changing the cylinder address.
A device related to this type of arrangement for seek operation control by using the magnetic head with the IGB and OBG may be seen in, for example, British Pat. No. 1435368.